The Average Recurrence Interval (ARI) of a rain event is an estimate of the average time between random events. For example, in relation to rain events, historical data may indicate that a particular area has had 50 mm of rain in 30 minutes approximately every 25 years. This means that the probability of 50 mm of rain falling in 30 minutes in a particular locality in any one year is 1 in 25. Alternatively, there is a 4% chance of such a rain event in any one year, or 4 AEP.